The present invention generally relates to charged particle beam exposure methods, and more particularly to a charged particle beam exposure method which uses a charged particle beam such as an electron beam to selectively expose a resist layer.
Ultraviolet our photolithography was mainly used for forming fine patterns. But recently, due to further increases in the integration density of integrated circuits, new exposure techniques have been developed and and put into practice. The new exposure techniques use charged particle beams such as X-ray beams or electron beams.
Charged particle beam exposure techniques enable the formation of patterns with beams that can be controlled electromagnetically. One notable feature of charged particle beam exposure techniques is the fact that it is thereby possible to form fine patterns of the submicron order. Charged particle beam exposure techniques can generally be categorized into Gaussian beam exposure methods and shaped beam exposure methods.
The trend to further increase the integration density of integrated circuits is rapidly accelerating. As compared to optical beams, the diameter of the minimum spot which can be formed using electron beams is far smaller than that achievable by the optical beam. For this reason, electron beam exposure techniques fully satisfy the demand for increased integration density from the point of view of resolution.
In Gaussian beam exposure methodology, the electron beam is converged to a small spot of a Gaussian distribution and the pattern is drawn by scanning with the spot. The exposure is made by a single strokes of the spot and the time it takes to complete the exposure over a large area becomes longer as the spot is made smaller.
An electron beam can be shaped into fine patterns and the patterned electron beam can be projected within a certain range. The use of such patterned electron beams can considerably increase the speed of forming the desired patterns on the resist layers. The shaped beam exposure techniques such a shaped electron beam. For example, the electron beam may be shaped into a beam with a variable rectangular cross section. A shaped beam with the variable rectangular cross section may be formed by two stages of beam shaping. The electron beam is initially passed through a first square aperture in a first stage, and the shaped electron beam formed in the first square aperture stage is then passed through a second square aperture in a second stage. The variable rectangular cross section of the electron beam can be controlled by the first and second square apertures. There are also proposals for forming the beam into oblong, square, and triangular shapes, and the like, by programming a combination of variable rectangles so that the desired shape can be formed by selecting a program. However, extremely large numbers of variable rectangles are required to achieve complex shapes by combining variable rectangles and it takes that much more time to complete the exposure.
The electron beam itself can be shaped with sufficient accuracy. But when the electron beam is irradiated onto a photo (electron beam) resist layer, forward scattering of the electron beam within the resist layer occurs to a certain extent. Further, when the electron beam hits the base layer, which is made of aluminum, silicon or the like, the electron beam is reflected and further scattering in the form of back scattering occurs in the resist layer. In this case, the backward scattering occurs over a wide range. The spread of the electron beam by scattering can be approximated by the Gaussian distribution as shown in the following formula. EQU F(r)=c1 exp[-(r/d1).sup.2 ]+c2 exp[-(r/d2).sup.2 ]
In the above formula, the first term corresponds to the forward scattering, and the second term corresponds to the backward scattering, where r is the distance from the center of the electron beam spot and the parameters c1, c2, d1 and d2 are dependent on the material of the base layer, the kind of resist layer, the acceleration voltage of the electron beam and the like. For example, when a polymethyl methacrylate (PMMA) layer is formed to a thickness of 0.5 microns on a silicon substrate and an electron beam having an acceleration voltage of 20 kV is used, the forward scattering d1 is approximately 1 micron and the backward scattering d2 is approximately 3 microns.
As the integration density increases and the intervals between adjacent exposure patterns become small, the spread of the electron beam caused by the scattering overlaps and affects adjacent patterns, thereby introducing the so-called proximity effect.
The proximity effect is described in conjunction with FIGS. 1A and 1B. In FIG. 1A, when the resist is exposed by an electron beam 111 which has a half-width corresponding to the width d for a separated independent design pattern 101 which has the width d, it is assumed that a developed pattern 121 having the width d is obtained. When the resist is exposed by electron beams 112 and 113 having half-widths which correspond to the width d for closely adjacent design patterns 102 and 103, each having the width d, a synthetic electron beam exposure 114 is obtained at the mutually confronting portions of the design patterns 102 and 103 where the bases of the electron beams 112 and 113 overlap. As a result, the central portion between the design patterns 102 and 103 exceeds the developing level for the resist, and the two design patterns 102 and 103 connect to become exposed as a single pattern 124. When two independent design patterns connect, this results in short-circuiting and the like within the semiconductor device which is produced.
It is possible to raise the developing level as shown in FIG. 1B so that the two peaks of the synthetic electron beam exposure 114 are separated and the two design patterns 102 and 103 are developed as independent patterns 122 and 123. However, even if were possible to adjust a gap g between the two patterns 122 and 123 to the designed value, the width of the pattern 121 or the patterns 122 and 123 becomes smaller than the design value d and the positions of the closely adjacent patterns 122 and 123 also change slightly. When the width of the pattern is smaller than the design value, this results in an increase in the resistance of the conductor path, and conductor breakage occurs due to over heating and the like within the semiconductor device which is produced. On the other hand, changes, in the position of pattern components result in poor contact at contact portions, and increases in stray capacitance and the like within the semiconductor device which is produced.
The proximity effect is the above described phenomenon whereby the dimensions or positions of components of the design pattern are altered due to the presence of the closely adjacent patterns which have an effect on one another.
FIGS. 2A through 2C are diagrams ofr expalining conventional methods of reducing the effects of the proximity effect.
FIG. 2A is a diagram explaining the adjustment needed to compensate for the reduced dimension and position shift. It is assumed that the design patterns 101, 102 and 103 have the same width d and that the two design patterns 102 and 103 are closely adjacent to each other. When the two closely adjacent design patterns 102 and 103 are treated similarly as the design pattern 101, the width of the closely adjacent patterns increases as shown in FIG. 1A and the closely adjacent patterns connect in some cases as a result of the proximity effect. Hence, the first method decreases the width of the closely adjacent patterns depending on the closeness of the closely adjacent patterns. But since the side of the pattern facing the outside is less affected by the proximity, the adjustment is made so that the side of the pattern facing the outside is more separated compared to the side of the pattern facing the inside. In other words, the dimension of the pattern is reduced and the position of the pattern shifts. When the exposure is made by the electron beam, the electron beams 112 and 113 overlap each other at the bases thereof, and the synthetic electron beam 114 develops so that the patterns 122 and 123 have the designed width at the designed position.
FIG. 2B is a diagram for explaining the making of corrections by altering the exposure or intensity of the electron beam. By this method the intensity of the electron beam for the closely adjacent patterns 102 and 103 is reduced. As a result, the synthetic electron beam 114 can maintain approximately the correct interval. But because the intensity of the electron beam is also reduced for the side of the pattern which faces the outside and is less affected by proximity offset, the width of the pattern slightly decreases. In order to maintain the designed width, the position of the pattern is changed slightly. For this reason, this second method is relatively simple when a simple pattern is being formed by Gaussian beam exposure but creates problems in that it is difficult to use this method to accurately reproduce both the designed position and width of the pattern.
FIG. 2C is a diagram for explaining corrections using the ghost exposure method. This method employs a ghost image to expose complementary portions than the design patterns 101, 102 and 103 at an intensity lower than that used for the main exposure of design patterns 101, 102 and 103. Normally, when the intensity of the main exposure is denoted by "1", the intensity of the ghost exposure is in a range of "0.2" to "0.5". A synthetic electron beam exposure 118 which is a combination of the ghost exposure 116 and the main exposure 117 covers the entire surface. At a portion where the patterns are closely adjacent, the patterns affect each other due to the proximity effect. On the other hand, at portions where each pattern is independent and separated from other patterns, the ghost exposure affects the main exposure of the pattern. The added exposure caused by the ghost exposure is small at portions where the patterns are densely arranged because the areas other than those occupied by the patterns are small. In other words, when the differences in the intensities of the main and ghost exposures are not taken into consideration, the same exposure level affects all of the patterns. Even when the difference in the intensities of the main and ghost exposures are taken into account, it is possible to affect all of the patterns by approximately the same exposure level, and the resulting patterns are very accurate. Accordingly, patterns having the desired dimensions as a whole are obtainable. However, the ghost exposure method uses a complete inverted pattern of the original pattern and thus suffers from the fast that the data quantity of the inversion pattern is extremely large and it takes a long time to make the ghost exposure.
A 16-Mbit dynamic random access memory (DRAM) is an example of a semiconductor device which requires super fine patterns. But although the patterns of the DRAM are super fine, most of the areas exposed contain a repetition of the same pattern. Hence, there is a possibility that accurate patterning can be achieved if this repetition of the same pattern is effectively utilized.
It is conceivable to prepare a transmission mask of a basic pattern which becomes a unit of repetition of the patterns, and repeat one-shot exposures using this mask in order to expose the repetition of the same pattern. For example, the basic pattern which is formed in the mask may correspond to one or several cells of the DRAM, one or at least a portion of several cells of a static RAM (SRAM) or the like. The drawing of the repetition of patterns is made by repeating the exposure of the basic pattern from head to tail in single shots and connecting the patterns obtained by each exposure.
The proximity effect is large in the case of the repetition of patterns which are dense. It is possible to consider the proximity effect and shift the dimensions within a single basic pattern. Further, when the basic pattern is repeated to form a large pattern, it is possible to consider the proximity effects on the basic pattern which is located at the central portion caused by the basic patterns located at the periphery. However, although the basic pattern located at the central portion is surrounded by the basic patterns which are located at the periphery, there are no basic patterns surrounding the basic pattern which is located at the periphery. In other words, the proximity effects differ depending on the location of the basic pattern. It is impossible to compensate for the difference in the proximity effects by shifting the dimensions within the basic pattern as long as the same mask is used to expose the basic pattern repeatedly.
The exposure intensity adjustment method can conceivably be employed for each line in the above situation, provided that the basic pattern is formed by a single line and data is prepared beforehand for adjusting the intensity of the exposure depending on the exposure position. However, this exposure intensity adjustment method cannot be employed when the basic pattern is formed by a plurality of lines.
In principle, the ghost exposure method is effective in reducing the proximity effect. However, the number of exposure steps and the exposure time each increase because of the need to also expose all of the non-pattern areas.
Therefore, conventional technology does not provide a method of effectively preventing or reducing the proximity effect by a simple process which does not take much time especially when the patterns are repetitions of basic patterns.
Furthermore, problems similar to those described above also occur when producing reticles and masks.